A number of individual superbubbles exhibit multi-phase ISM, as is
qualitatively predicted by the adiabatic shell model.
DEM L152 (N44) in the Large Magellanic Cloud (LMC) is a beautiful example
where the nebular (104 K) gas in the shell clearly confines the
hot, X-ray-emitting (106 K) gas within
(Magnier et al. 1996;
Figure 4).
Chu et al. (1994)
also confirmed the existence of C IV and Si IV absorption
in the lines of sight toward all stars within LMC superbubbles. These
tracers of intermediate temperature (105 K) gas are expected in
interface regions between hot and cold gas. Quantitatively, however,
the detected X-ray emission from LMC superbubbles has been an order of
magnitude higher than predicted by the adiabatic model. It is
therefore thought that the anomalous emission results from impacts to
the shell wall by internal SNRs
(Chu & Mac Low 1990;
Wang & Helfand 1991),
a scenario which is supported by other signatures such as
enhanced [S II] / H and
anomalous kinematics
(Oey 1996;
see below).
Many other superbubbles have not been detected in X-rays, although the
upper limits tend to be high, and remain within the model predictions.
It is hoped that the capabilities of Chandra and XMM will
be applied to these objects.

Since most early-type stars are found in OB associations, wind-driven
bubbles of individual massive stars are rare, and consequently few
quantitative studies of these objects exist. In principle, O stars
offer the most straightforward test of the standard shell evolution,
since their wind histories are simple and relatively well-understood.
One of the few such studies was carried out by
Oey & Massey (1994) on
two nebular bubbles around individual late-type O stars in M33. They
found crude consistency with the model predictions, but the constraints
are limited by lack of kinematic information. Cappa and collaborators (e.g.,
Cappa & Benaglia 1998;
Benaglia & Cappa 1999;
Cappa & Herbstmeier 2000)
have studied a number of H I shells around Of
stars, which are presumably evolved O stars. They generally find a
significant growth-rate discrepancy such that the shells appear to be
too small for the assumed stellar wind power. Wolf-Rayet (W-R) stars
are well-known to have the most powerful stellar winds, and a number
of W-R ring nebulae have also been examined kinematically. Optical
studies include those by
Treffers & Chu (1982),
García-Segura & Mac
Low (1995),
and Drissen et al. (1995);
while, e.g.,
Arnal et al. (1999)
and Cappa et al. (2002)
examine the neutral and radio continuum properties.
The W-R nebulae are also apparently too small, but owing to the
complicated stellar wind history and associated environment, it is
more difficult to interpret the W-R shell dynamics.

Superbubbles around OB associations are much larger, brighter, and
easier to identify than single star wind-driven bubbles, and
consequently have been much more actively studied. They are
especially prominent in the LMC, where the proximity and high galactic
latitude offer a clear, detailed view of the objects. The most
comprehensive study of superbubble dynamics was carried out by Oey
and collaborators
(Oey 1996;
Oey & Smedley 1998;
Oey & Massey 1995)
on a total of eight LMC objects.
Saken et al. (1992) and
Brown et al. (1995)
also examined two Galactic objects. All of these are
young, nebular superbubbles having ages
5 Myr. These
studies again consistently reveal a growth-rate discrepancy equivalent
to an overestimate in the inferred L / n by up to an order
of magnitude. About half of the objects also show anomalously high expansion
velocities, implying a strong, rapid shell acceleration from the
standard evolution.

A number of factors could be individually, or collectively,
responsible for these dynamical discrepancies. The first possibility
is a systematic overestimate in L / n: stellar wind parameters
remain uncertain within factors of 2 - 3. The ambient density
distribution is also critical to the shell evolution; as shown by
Oey & Smedley (1998),
a sudden drop in density can cause a
"mini-blowout" of the shell, whose kinematics can reproduce those of
the high-velocity LMC shells. In the case of those objects, however,
the presence of anomalously high X-ray emission favors the SNR impact
hypothesis discussed above. In any case, the critical role of the
ambient environment motivated us to map the
H I distribution around
three superbubbles in the LMC sample
(Oey et al. 2002).
The results
were surprisingly inhomogeneous, with one object essentially in a
void, another with significant H I in close
proximity, and a third
with no correspondence at all between the nebular and neutral gas.
Thus it is virtually impossible to infer the ambient
H I properties
for any given object without direct observations. This heterogeneity
suggests that a systematic underestimate of n is not responsible for
the universal growth-rate discrepancy. However, a related
environmental parameter is the ambient pressure. If the ISM pressure
has been systematically underestimated, then the superbubble growth
would become pressure-confined at an earlier, smaller stage. We are
currently exploring this possibility
(Oey & García-Segura 2003,
in preparation).

Finally, if the superbubble interiors are somehow cooling, then
the shells will no longer grow adiabatically. While this possibility
has been explored theoretically from several angles, there is as yet
no empirical evidence, in particular, radiation, that the objects are
cooling. Meanwhile, mass-loading has long been a candidate cooling
mechanism (e.g.,
Cowie et al. 1981;
Hartquist et al. 1986),
either by
evaporating material from the shell wall, or by ablating clumps
that are overrun by the shell. The enhanced density would then
increase the cooling rate of the hot interior. More recently,
Silich et al. (2001)
and Silich & Oey (2002)
suggested that the metallicity
increase expected from the parent SN explosions can significantly
enhance the cooling and X-ray emission, especially for extremely low
metallicity systems.